Flow International Corporation
5-axis gantry robots and 6-axis articulated arm robots have been used with plain waterjets for many applications especially in the automotive industry. This paper is on extending the use of these robots to abrasive waterjets and for a much wider range of applications. The integration of the abrasive waterjet process on robotic arms has been successfully developed to address the end effector, supply of high pressure water and abrasives to the cutting head, and operational safety. Off line programming, calibration, and inspection are discussed. Advanced software packages typically used in the aerospace industry have been successfully adapted. The need for enhanced accuracy performance using first article inspection results is discussed. A few case studies are presented in this paper addressing composite trimming for wing skins used in aircraft and wind turbines and for stone cutting with hybrid processes. It was found that 6-axis robot arms can easily be implemented when moderate accuracies, from 0.010 inch (0.25 mm) to 0.015 inch (0.38 mm) are specified. However, accurate calibration and first article inspection procedures were found necessary to obtain accuracies below 0.01 inch (0.25 mm). Much tighter accuracies are achieved with stiff gantry systems.
Today, thousands of multi axis robotic cells are deployed worldwide in a variety of high production environments. These include; manufacturing of standard and customized components, prototype designs, and low & high volume mass production of large and medium batch sizes for OEM and aftermarket sectors. In these applications, the robot accuracy and repeatability have been sufficient.
For accurate processing of the workpiece and especially cutting with a beam-like cutting tool such as waterjet and abrasive waterjet (AWJ), more understanding of all aspects and process variables that affect accuracy is necessary. These variables can stack up and result in out-of-tolerance parts. These variables could be related to the manipulator, fixture, material, tool/process, and the environment. The robot, for example is a complex assembly of interconnected links, gears, gear trains, servo drives, harmonic drives and even belt drives. These result in positional and path errors which need to be compensated for. Also, recognizing the idiosyncrasies of the material, such as homogeneity, shrinkage, warping, etc. will lead to minimized errors by proper compensation (1-3)
In this paper, we first address manipulators for waterjet applications covering gantry and 6-axis robots. Then, we will address AWJ applications such as trimming, cutting, and shaping using 6-axis robots with focus on composite trimming. Conclusions are listed at the end of the paper.
In this section we address 5-axis gantry robots and 6-axis robotic arms because they are the most used in multi-axis waterjet cutting.
The 5 axis gantry systems represented by Figure 1, are Cartesian manipulators and are designed with relatively high degree of structural stiffness in order to achieve the required accuracy. It is typical in the design of these systems to study their structural, kinematic, and dynamic behaviors using analysis software packages such as finite elements. For example, Figure 1 (right) shows the results of a finite element analysis used to determine the deflection of a gantry bridge under given loads. The analysis may also include thermal deformation and whether environmental control will be required or not based on the required part accuracy. The deflection data can be used for compensation in order to obtain more accurate results.
These gantry units are sometimes installed on a machine tool foundation and housed in a controlled environment to maximize stiffness and minimize the thermal effects. When mechanical end effectors such as routers are integrated into waterjet gantry systems, the stiffness requirement becomes the driving factor for machine construction. Helical rack and precision gear boxes and closed loop control systems contribute to smooth motion and high accuracy. Before using these machines, the linear axes are compensated using laser interferometers. Linear encoder scales are also used to increase the accuracy on large scale systems. Figure 2 shows a picture of a typical 50-m long hybrid waterjet-router gantry system used for trimming wing skins for the Boeing 787 (4).
6-axis articulated arm robots have been used in a very wide range of industries such as automotive, electronics, and entertainment due to their flexibility, relatively low cost, and reduced footprint. These robotic arms can also be mounted in a variety of ways based on the application such as floor mount, ceiling mount, gantry mount, wall mount, etc. as shown in Figure 3.
Although highly repeatable, robots are not accurate until they are “mastered”, i.e., compensated. They are typically manually taught by eye from a teach pendant, Figure 4, using a ‘point-to-point’ format. The programmer may choose to use a custom pointer as the simulated Tool Center Point (TCP) or a peripheral laser pointing device, aimed at the target zone or they may simply guess and cut a part, measure the deviations and correct it with another pass and so on. During this procedure the operator will program the path, consisting of lines and circles and also define speeds, corner positioning etc. and assign any I-O (inputs and outputs) to control or monitor external functions. These external devices can be the nozzle, vacuum system, clamps etc. The robot can also be oriented in a variety of positions to suit the work environment. Moving forward from this manual procedure to an offline programming environment will enable elimination of the teach pendant. This will require a custom post processor and process specific offline programming software relative to the particular brand of robot. In order to do this, the robot, the end effector TCP and the work holding fixture must be defined in the real world. As with the gantry systems, robot rigidity is also a very important factor to ensure high accuracy calibration.
Robots are integrated in several system styles such as open and enclosed systems, Figure 5. Also, more than one robot can be used on the same part. In this case, off line programming will be critical.
Waterjet robotic cutting has been early introduced due to the advantages of both robots and waterjet. For example waterjets (without abrasives) have been used for cutting and trimming a wide range of automotive interior materials (5-7). Among these are:
• Floor carpets and roof liners.
• Dash sound insulation material.
• Plastic/Fabric composites (i.e. rear shelf and transmission tunnel components)
• Sheet molded composites (SMC)
• Glass fiber components (body panels etc.)
• Carbon fiber and /or Aramid fiber composite
Recently, abrasive waterjets (AWJ) were integrated with robots to capitalize on their flexibility. However, this introduced new significant challenges related to abrasive feed, accuracy, speed range, and safety. Figure 6 shows an example of an AWJ-equipped robot arm.
As discussed above, robots can be mounted in different ways based on the application. For AWJ use, floor mount AWJ systems have been commercialized. Also, Figure 7 shows alternative methods to using robotic AWJ arms either on a stationary frame or on a single axis gantry.